Automatic defrosting control system

ABSTRACT

An automatic defrosting control system for a refrigeration system having an evaporator or the like for absorbing heat from a zone to be cooled and a cold control to maintain the zone substantially at a preselected temperature. The control system comprises means for causing defrosting of the evaporator and two thermostats.

[451 Aug. 19, 1975 AUTOMATIC DEFROSTING CONTROL SYSTEM Inventor:

Armentrout Harbour........ 970 West.........

[75] Samuel T. Bryant, Louisville, Ky.

[73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

[22] Filed: Oct. 15, 1974 Primary Examiner-William J. Wye

Attorney, Agent, or Firm-James P. McAndrews; John A. Haug; Russell E. Baumann Appl. No.: 514,860

ABSTRACT An automatic defrosting control system for a refrigeration system having an evaporator or the like for abmwmm 2 12 W22 6 8 5 D wsm 6 2y 6 m 5 n5 5 5 H m M mh c u r mm l wf. C l0 .M t e U mm T UN 5 55 sorbing heat from a zone to be cooled and a cold control to maintain the zone substantially at a preselected temperature. The control system comprises means for causing defrosting of the evaporator and two thermosta References Cited UNITED STATES PATENTS 14 Claims, 7 Drawing Figures PATENTED AUG] 9 i975 sum 2 UP 2 F|Cl4 FIGE) This invention relates to a defrost system for various types of refrigeration apparatus, the defrost system being operable after periods of controlled length to deal with anticipated buildup of frost on the cooling unit (e.g., the evaporator) of the refrigeration apparatus during such periods.

The accumulation or build-up of frost on the evaporator of a refrigerator or other refrigeration unit has long been a problem. Various automatic defrosting systems have been used and are well known in the art. Typically, an automatic defrost system is controlled by a mechanical timer which initiates operation of the defrost system at certain times of the day or after the compressor has run a predetermined length of time. The rate at which frost forms on the evaporator is a function of the amount of water vapor in the air passing over the evaporator, the greater the water content the faster the frost accumulates. In a refrigerator, the amount of water vapor within the air to be cooled depends a great deal on the ambient conditions (i.e., room temperature and relative humidity) outside the refrigerator because ambient air is introduced into the refrigerator each time the door is opened and closed, and water vapor sources (e.g., wet produce and open containers of liquids) within the refrigerator. With defrost system's'controlled only with respect to time and with a slow build-up of frost, operation of the defrost system is sometimes initiated before any significant amount of frost has built up on the evaporator, thus resulting in a wastage of power to defrost the refrigerator when it is not required and exposing the items in the refrigerator to unnecessary defrost cycles. On the other hand, under heavy frost conditions, excessive frost may build up on the evaporator between the timed defrost cycles, thus reducing the efficiency of the refrigerator, increasing the power consumed thereby and warming foodstuff that should be kept cool, resulting in shorter shelf-life for refrigerated foods and possible contamination unknown to the user. Mechanical timers used in such systems have also tended to be somewhat expensive.

Another defrosting system is one in which the number of door openings are counted and a defrosting cycle is initiated after a selected number of openings occur. This arrangement is disadvantageous in that an unused or little used refrigerator would not be defrosted even though a substantial frost deposit has built up. Also, mechanical counters are relatively unreliable in continued use. Depressd temperature systems have also been utilized where defrosting cycles are initiated by detecting when the evaporator reaches a specific temperature much lower than its normal operating temperature. This depressed evaporator temperature occurs after ice forms on the evaporator, reducing its efficiency. Depressed temperature systems have not been too successful because the lower evaporator temperature varies from evaporator to evaporator due to production tolerances and good sensing of a specific depressed temperature as the sole basis for initiating a defrost cycle has been difficult due to inconsistency of heat transfer materials used between the evaporator and the sensing control. Depressed temperature systems have, as a rule, been'more expensive than the systems in current usage.

Other systems utilized have been restricted air-flow methods with electronic sensors, but these are relatively expensive and difficult to build in production. Fluidic systems initiating defrost based on pressure changes in the refrigerating equipment are also expensive.

SUMMARY OF THE INVENTION Among the several objects of this invention may be noted the provision of an automatic defrost system for refrigeration apparatus (e.g., a refrigerator, a freezer, a refrigerated vending machine, or an air conditioner) in which a defrosting cycle is initiated after a period of time, the length of which is related to the occurrence of conditions which would tend to build up fast on the evaporator rather than having defrosting initiated by a clock or counter mechanism alone; the provision of a defrost system which conserves power and increases the operating efficieny of the refrigeration apparatus by eliminating unnecessary defrost cycles and by keeping the refrigeration apparatus free of excessive frost; the provision of such a defrost system which maintains better temperature control in the refrigeration system and which does not expose refrigerated or frozen items to unnecessary defrost cycles; the provision of defrost systems of the class described which permit a fast cool down of warm foods, fast freezing of foods and increased ice production; the provision of such a defrost system which is relatively simple and of economical construction and which will reliably operate regardless of ambient climatic conditions. Other objects and features will be in part apparent and in part pointed out hereinafter.

Briefly, an automatic defrosting control system of this invention comprises means for causing defrosting of the refrigeration system cooling means and two thermostats. The first thermostat is positioned adjacent and in heat-exchange relation with the defrosting and cooling means and is connected in a control circuit. This thermostat has a first switching position for terminating operation of the defrosting means and a second switching position for enabling operation thereof. The first thermostat switches from its first to its second position in response to its temperature falling to a predetermined level and switches back to its first position in response to its temperature rising to a predetermined level. The second thermostat is positioned adjacent the cooling means but in a poorer heat-exchange relation therewith than is said first thermostat. The second thermostat is also connected in the control circuit and has a first switching position for permitting energization of the cooling means and a second switching position for energizing the defrost means while preventing operation of the cooling means. The second thermostat switches from its first to its second switching position in response to its temperature falling to a predetermined level. After defrosting, and during the period when frost tends to build up on the cooling means, the temperatures of both thermostats are reduced from a relatively high temperature established during defrosting until, after a first time delay, due to the thermal lag between the temperature of the cooling means and that after cools sufficiently to switch to its second position thereby energizing the defrosting means. In this way, defrosting is initiated after a period of time without the use of an expensive mechanical timer, and the length of such period is related to the cooling conditions under which frost build up would tend to occur.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view in section of refrigeration apparatus employing a defrost system of this invention and illustrating the normal flow of air therethrough when the refrigeration apparatus is in operation;

FIG. 2 is an enlarged semidiagrammatic view of a bracket mounting the thermostats on the evaporator;

FIG. 3 is a circuit diagram of a defrost system of this invention;

FIG. 4 is a graphical representation of the temperatures of various components of the refrigeration system and the thermostats during operation beginning with a frost-free condition after a defrost period, through normal operation where frost accumulation would tend to occur, to a point where the frost deposit would tend to be substantial enough to require initiation of defrosting; and

FIGS. 5-7 are schematic circuit diagrams of other embodiments of automatic defrost systems of this invention.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, a defrost system of this invention, as indicated generally at 1, is shown installed in a conventional two-door refrigeratonfreezer 3. The refrigerator-freezer includes a cabinet 5 having a top 7, side walls 9, a back wall 11, a bottom wall (not shown) and a partition 13 dividing the interior of the cabinet and defining a freezer compartment 15 and a food compartment 16, these compartments constituting refrigerated zones. A freezer door 17 and refrigerator door 19 close the front of the cabinet. The refrigerator includes a conventional refrigeration system including a compressor driven by an electric compressor motor C (see FIGS. 3 and 5-7), a condenser (not shown) and a cooling unit or evaporator generally indicated at 23. The evaporator includes a plurality of refrigerant lines 25 constituting a coil, this coil being subject to frost build-up. A flow path generally indicated at 27 provides for the intake of air from both the freezer and food compartments, for the passage of this air over the evaporator for absorbing heat from the air and thus chilling the air, and for the discharge of the chilled air into the refrigerated compartments or zones. A blower or fan 29 is provided for forcing air through the flow path. While the defrost system of this invention is depicted as installed in a two-compartment refrigerator-freezer, it will be understood that it may be installed in other refrigeration apparatus, such as a sin gle-compartment refrigerator, a freezer, a refrigerated vending machine, or an air conditioner.

More particularly, flow path 27 is, in part, defined by partition 13 and by a horizontal panel 31 in freezer compartment 15 spaced above the partition and thus forming a main passage 33 between the horizontal panel and the partition. An opening 35 is provided in partition 13 for the intake of air into the passage from food compartment 16 and an opening 37 is provided in panel 31 for the intake of air from the freezer compartment. Evaporator 23 is located within passage 33 for chilling air from the food and freezer compartments as it passes thereover. An inner vertical panel 39 is spaced from back wall 11, thereby to provide a return or outlet passage 41 for the discharge of chilled air into the food compartment via an outlet 43. A vertical wall 45 extends up from panel 31 and a fan shroud 47 is disposed between vertical wall 45 and panel 39, thereby to define a fan intake chamber 49 and a discharge chamber 51, with the upper end of the fan intake chamber being closed by a cap 53. An opening 55 in panel 31 provides communication between main passage 33 and the fan inlet chamber. A baffle 57 directs and divides the chilled air discharged from the fan into outlet passage 41 for discharge into the food compartment and into freezer compartment 15 via openings 58 and 59.

The defrost system 1 of this invention comprises a first or defrost termination thermostat T1 and a second or defrost initiation thermostat T2 mounted on a bracket B, both of which are connected in a circuit for controlling energization of a heater DH for melting and removal of frost from evaporator 23 upon a predetermined build-up of frost thereon. Bracket B, as indicated in FIG. 2, is positioned in heat-exchange relation with both the heater DH and the evaporator by being secured to a frame 61 for the coils of evaporator 23 by means such as a thermal union 63. Union 63 may constitute any conventional joint or means for physically affixing the bracket to the coil whereby the desired degree of heat conductivity is achieved. For example, the bracket may be directly bolted, clipped, cemented, or taped to the frame with the thermal conductivity of the joint limited by spacers, reduced sections, etc. Also, insulating or conductive materials and synthetic resins may be employed in this union. Thermostats T1 and T2 are conventional temperature-responsive widedifferential switches, such as any of the widely used bimetallic disk-actuated types in which the contacts are abruptly moved from one switching position to the other when heated and cooled.

T1 is mounted in closer heat-transfer relationship to evaporator 23 than is T2, so that the latter will have poorer heat-exchange relationship therewith or have a greater lag in its temperature relative to the evaporator temperature than will Tl. For example T2 may be mounted in a position on the bracket B more distant from the evaporator than T] is mounted. Or the bracket may be perforated or have a reduced section between the two spaced mounting positions of TI and T2. These thermostats and bracket are preferably mounted out of the air flow across the evaporator to minimize convection cooling and may be conveniently mounted within an enclosed insulated well in the side wall of the refrigerator.

Referring now to FIG. 3, thermostats TI and T2 are schematically shown connected in a control circuit for selectively energizing defrost heater DH and periodically actuating compressor motor C in response to the temperature sensed by thermostat T3, a conventional adjustable cold control typically positioned in refrigerator food compartment 16. Optionally, but preferably, included in the control circuit is a heater comprising a resistor H together with a second heater R, also constituted by a resistor. Resistors R and H are for example self'regulating. self-heating positive temperature coefficient resistors which have a relatively low resistance when deenergized at ambient temperature but which will increase in resistance abruptly as their temperature rises above a given level. Heater H is positioned in close heat-exchange relation with T2 (as indicated by the dashed line therebetween indicating a thermal link) and is preferably enclosed within the housing thereof. Heater R is located so as to transfer heat to bracket B.

Tl will move from a first (solid-line) switching position, in which operation of the defrost heater DH is terminated, to its second (broken-line) position when its temperature falls to a level of F, for example, and will not switch back to its first position until its temperature rises to say 60F. Similarly T2 will move from its first (solid-line) switching position, in which the compressor may be energized, to its second (broken-line) position, in which the heater DH may be energized, only when its temperature falls to OF., remaining there until its temperature rises to IOOF. whereupon it abruptly reverts to its first position.

As illustrated in FIG. 3, and with thermostats TI and T2 both in their first or solid-line positions, the defrost heater DH is disabled and compressor C will be periodically energized each time cold control thermostat T3 moves to its solid-line position in response to the food compartments temperature rising above a selected control temperature level. Resistor heater R and H will be series-connected across an ac. power source indicated at Ll,L2 each time T3 closes. This is the normal operational mode of the circuit between termination of one defrost mode and the initiation of the next.

FIG. 4 illustrates the temperatures of the various system components, each noted parenthetically in relation to its respective temperature curve, beginning with the termination of a defrost mode. At that moment T3 will be closed ready to initiate operation of the cooling means C, T2 will be in its second or broken-line position, and TI, having been heated to its 60F. temperature by the defrost heater DH, will have just switched to its solid-line position (thereby disabling heater DH). The evaporator tubing 25, frame 61, the thermostat T1, and bracket B will all be at their maximum temperatures to which they were heated by DH. As heater H for T2 has been deenergized during defrosting (while T1 was in its broken-line position), T2 will still be below the 100F. temperature level to which it must rise to switch back to its first or solid-line position. However, heater H is reenergized as T1 switches to its solid-line position and, heater H will begin heating T2 as indicated at the top left of FIG. 4. The time required for H to heat T2 to 100F. provides a short period of additional time after heater DH is deenergized for permitting the melted frost to drain from the evaporator and frame before T2 switches to'its solid-line position and initiates reenergization of compressor C. The refrigerator normal apparatus is now in its operational mode causing the illustrated temperature decreases in coils and frame 61 as the compressor C goes into its initial on mode. The normal operational mode of the refrigerator apparatus then continues as regulated by the cold control thermostat T3 with each on and off mode of the compressor C causing the thermal swings in the evaporator coils 25 and the frame 61 as shown in FIG. 4. The air wiping across the evaporator warms the frame 61 somewhat relative to the evaporator coils as illustrated in FIG. 4. however, as indicated,

bracket B and thermostat Tl will not follow the thermal swings of the coils and the frame. That is, because of the thermal union, the thermal mass of the bracket and the thermostats themselves, and the intermittent energization of heaters H and R each time the cold control thermostat T3 closes, the temperatures of T1, T2 and the bracket follow different patterns or gradients. Because of the thermal capacitance of the mounting system, there is a thermal opposition to the cooling effect of the evaporator so that there is a thermal lag or phase difference between the temperature of the termination thermostat T1 and the evaporator coils and a second and greater thermal lag or delay between the temperature of the initiation thermostat T2 and that of the evaporator. Also, as heaters H and R are energized during each on mode of the compressor, T2 and bracket B will be supplied with heat during these periods, further delaying their cooling to a significantly lower temperature.

As the normal operational mode continues and frost tends to accumulate on the evaporator coils and frame, a continued incremental cooling of TI occurs until it reaches its lower temperature, e.g. 20F, as indicated at X in FIG. 4 and switches to its second (broken-line) position. However, while this enables operation of heater DH, energization of DH will not occur as T2 still remains in its first (solid-line) position with its temperature considerably higher than that of TI. The resistance of R is such that no significant current will be supplied to DH via R under these conditions. The switching of T1 into its second position deenergizes heater H so that gradually, as indicated in FIG. 4, the temperature of T2 will fall until its temperature drops to 0F. as indicated at Y. T2 will then switch to its second (broken-line) position to shunt heater R and apply full line voltage to heater DH thereby initiating a defrost cycle. It should be understood that the curves of FIG. 4 are merely representative or illustrative and that many more on and ofF cycles of the compressor would take place between a termination of one defrost mode and the initiation of the next one.

During the defrost mode, T3 continues to remain in its closed position and T2 in its broken-line position. After the accretion of frost is melted by DH, the temperature of the evaporator, frame, bracket and T1 will rise until the temperature of the latter increases to F. This termination thermostat, which also provides a safety function in preventing overheating of the unit in case of a circuitry fault, will then switch to its first or solid-line position thereby completing one full cycle of operation of the automatic defrosting system of this embodiment of the invention.

It will be noted that the particular switching temperatures referred to above are merely illustrative and they may be varied widely within broad limits.

Another embodiment of the present invention is schematically illustrated in FIG. 5 wherein the defrost heater DH is serially connected with thermostat T3, heaters R and H and thermostat T1. Operation is substantially the same as that of FIG. 3 described above except that heater H is energized through DH and R when T3 closes and TI and T2 are in their normal operating or solid-line positions. However, as DH has a relatively low resistance, this will not significantly affect the circuit operation. Again, as in FIG. 3, the moving of the contacts of T1 and T2 into their broken-line positions deenergizes R (by shunting action of T2) and H (by switching of T1) and applies full line voltage to DH to initiate defrosting.

The alternate embodiments of FIGS. 6 and 7 also operate in a fashion similar to those of FIGS. 3 and 5, but differ in utilizing a single-pole single-throw thermostatic switch TlA rather than the singlepole doublethrow thermostat Tl. Optional heater R is not utilized in the FIG. 6 control circuit while in FIG. 7 heater R is serially connected with H and DH across L1 and L2 with each being deenergized by the shunting action of T3,T2 (as to R) and that of T1 (as to H) when both thermostats are in their second (broken-line) positions during each defrost mode.

It will be understood that the wattage, type and placement of optional resistor heaters R and H may be changed to vary the temperature gradients.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An automatic defrosting control system for a refrigeration system having cooling means for absorbing heat from a zone to be cooled and thermostatic means for periodically energizing the cooling means to maintain the zone substantially at a preselected temperature, said cooling means being subject to the accretion of frost thereon, said control system comprising:

means for causing defrosting of said cooling means;

a first thermostat adapted to be positioned adjacent and in heat-exchange relation with both said defrosting and said cooling means and adapted for connection in a control circuit, said first thermostat having a first switching position for terminating operation of said defrosting means and a second switching position for enabling operation of said defrosting means, said first thermostat switching from its first to its second position in response to its temperature falling to a predetermined level and switching to its first position in response to its temperature rising to a predetermined level; and a second thermostat adapted to be positioned adjacent said cooling means but in a poorer heat-exchange relation therewith than is said first thermostat, said second thermostat adapted for connection in said control circuit, said second thermostat having a first switching position for permitting energization of said cooling means and a second switching position for energizing said defrost means and preventing operation of the cooling means, said second thermostat switching from its first to its second switching position in response to its temperature falling to a predetermined level whereby as frost tends to build up on the cooling means the temperatures of both thermostats are reduced until after a first time delay due to the thermal lag between the temperature of said cooling means and that of the first thermostat the latter is cooled sufficiently to switch to its second position and after a second and longer time delay, due to the greater thermal lag between the temperature of the cooling means and that of the second thermostat, the second thermostat thereafter cools sufficiently to switch to its second position thereby energizing the defrosting means.

2. A system as set forth in claim 1 wherein the predetermined level to which the second thermostat must fall to switch to its second position is a lower temperature level than that to which the first thermostat must fall to switch to its second position. I

3. A system as set forth in claim 1 further comprising a heating means in heat-exchange relation with said second thermostat, said heating means adapted for connection in said control circuit for energization when said first thermostat is in its first position and for deenergization when said first thermostat is in its second position.

4. A system as set forth in claim 3 wherein said heater is a self-regulating, self-heating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level.

5. A system as set forth in claim 3 in which the cooling means includes an electrically energized compressor for connection across an electrical power source, said second thermostat when in its first switching position serially connecting said thermostatic means and the compressor across the a.c. power source, said thermostatic means being responsive to close therby to operate the compressor when the temperature in the zone rises above a preselected level.

6. A system as set forth in claim 5 in which the cooling means further includes an evaporator coil, and which further includes a bracket thermally associated with the evaporator, said first and second thermostats being mounted on said bracket with the thermal conductivity between the evaporator and the second thermostat being substantially less than the thermal conductivity between the evaporator and the first thermostat.

7. A system as set forth in claim 6 which further includes a second heating means serially connected with the first heater means for energization when the second thermostat is in its first switching position, said second heating means being positioned in heat-exchange relationship with said bracket thereby to heat it when energized.

8. A system as set forth in claim 7 wherein the first and second heater means are serially connected with the thermostatic means for energization only when the latter means is closed.

9. A system as set forth in claim 7 wherein said heating means are resistors.

10. A system as set forth in claim 9 wherein at least one of said resistors is a self-regulating, self-heating positive coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level.

11. A system as set forth in claim 7 wherein the second heating means is shunt-connected across contacts of said second thermostat whereby when the latter is in its second switching position the second heater means is shunted and deenergized and the energization level of the first heating means is increased.

12. A system as set forth in claim 5 in which said first thermostat is single pole-single throw and operates when in its second position to shunt said heater means to deenergize it.

perature to which said first thermostat must rise to switch from its second to its first switching position is lower than that temperature to which the second thermostate must rise to switch from its second to its first position. 

1. An automatic defrosting control system for a refrigeration system having cooling means for absorbing heat from a zone to be cooled and thermostatic means for periodically energizing the cooling means to maintain the zone substantially at a preselected temperature, said cooling means being subject to the accretion of frost thereon, said control system comprising: means for causing defrosting of said cooling means; a first thermostat adapted to be positioned adjacent and in heat-exchangE relation with both said defrosting and said cooling means and adapted for connection in a control circuit, said first thermostat having a first switching position for terminating operation of said defrosting means and a second switching position for enabling operation of said defrosting means, said first thermostat switching from its first to its second position in response to its temperature falling to a predetermined level and switching to its first position in response to its temperature rising to a predetermined level; and a second thermostat adapted to be positioned adjacent said cooling means but in a poorer heat-exchange relation therewith than is said first thermostat, said second thermostat adapted for connection in said control circuit, said second thermostat having a first switching position for permitting energization of said cooling means and a second switching position for energizing said defrost means and preventing operation of the cooling means, said second thermostat switching from its first to its second switching position in response to its temperature falling to a predetermined level whereby as frost tends to build up on the cooling means the temperatures of both thermostats are reduced until after a first time delay due to the thermal lag between the temperature of said cooling means and that of the first thermostat the latter is cooled sufficiently to switch to its second position and after a second and longer time delay, due to the greater thermal lag between the temperature of the cooling means and that of the second thermostat, the second thermostat thereafter cools sufficiently to switch to its second position thereby energizing the defrosting means.
 2. A system as set forth in claim 1 wherein the predetermined level to which the second thermostat must fall to switch to its second position is a lower temperature level than that to which the first thermostat must fall to switch to its second position.
 3. A system as set forth in claim 1 further comprising a heating means in heat-exchange relation with said second thermostat, said heating means adapted for connection in said control circuit for energization when said first thermostat is in its first position and for deenergization when said first thermostat is in its second position.
 4. A system as set forth in claim 3 wherein said heater is a self-regulating, self-heating positive temperature coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level.
 5. A system as set forth in claim 3 in which the cooling means includes an electrically energized compressor for connection across an electrical power source, said second thermostat when in its first switching position serially connecting said thermostatic means and the compressor across the a.c. power source, said thermostatic means being responsive to close therby to operate the compressor when the temperature in the zone rises above a preselected level.
 6. A system as set forth in claim 5 in which the cooling means further includes an evaporator coil, and which further includes a bracket thermally associated with the evaporator, said first and second thermostats being mounted on said bracket with the thermal conductivity between the evaporator and the second thermostat being substantially less than the thermal conductivity between the evaporator and the first thermostat.
 7. A system as set forth in claim 6 which further includes a second heating means serially connected with the first heater means for energization when the second thermostat is in its first switching position, said second heating means being positioned in heat-exchange relationship with said bracket thereby to heat it when energized.
 8. A system as set forth in claim 7 wherein the first and second heater means are serially connected with the thermostatic means for energization only when the latter means is closed.
 9. A system as set forth in claim 7 wherein said heating means are resiStors.
 10. A system as set forth in claim 9 wherein at least one of said resistors is a self-regulating, self-heating positive coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level.
 11. A system as set forth in claim 7 wherein the second heating means is shunt-connected across contacts of said second thermostat whereby when the latter is in its second switching position the second heater means is shunted and deenergized and the energization level of the first heating means is increased.
 12. A system as set forth in claim 5 in which said first thermostat is single pole-single throw and operates when in its second position to shunt said heater means to deenergize it.
 13. A system as set forth in claim 6 wherein the predetermined level to which the second thermostat must fall to switch to its second position is a lower temperature than that to which the first thermostat must fall to switch to its second position.
 14. A system as set forth in claim 13 wherein the temperature to which said first thermostat must rise to switch from its second to its first switching position is lower than that temperature to which the second thermostate must rise to switch from its second to its first position. 